1. Field of the Invention
The present invention relates to a network system constructed from a plurality of nodes, and particularly relates to an audio network system for transmitting audio signals. The present invention also relates to an audio signal transmitting system that transmits an audio signal from a transmitting device to a receiving device in real time. The present invention also relates to a method of detecting topology (network structure) in an audio signal transmitting system that transmits an audio signal from a transmitting device to a receiving device in real time, and to an audio network system with a function to detect topology.
2. Description of the Related Art
Routing (route control) is a technique to find a route for transmitting a packet of information between nodes on a computer network system constructed from a plurality of nodes. When a route is turned out, it is possible to transmit information by repeating transfer between nodes, that is, from an originating node to a node of a final destination along the route. In a conventional computer network system, each node holds a routing table for determining a route in accordance with an address of the destination, determines a route depending on an address added to a packet in accordance with this routing table, and transfers the packet along the determined route.
For example, in the general-purpose network system such as the Internet, in the case where a failure occurs on a route to transfer information (packet), it is possible to avoid the failure by dynamically controlling route information. Controlling route information dynamically is referred to as “dynamic routing”. The dynamic routing is a technique to automatically generate and update a routing table in each node, and to autonomously determine another route using existing nodes by each node. The dynamic routing is normally carried out by means of a routing protocol and path determination algorithm. The routing protocol is a protocol for exchanging routing tables between nodes each other. The path determination algorithm is algorithm for generating a routing table in each node. As the path determination algorithm, distance vector algorithm (hereinafter, referred to as “DVA”) and link state algorithm (hereinafter, referred to as “LSA”) are known.
The DVA is a method of determining an optimum route on the basis of “cost” recorded in a routing table that each node holds. The “cost” is a numerical value assigned to each node, and cost of a route connected between two nodes is expressed by the sum total of costs between nodes through the route. A list of destination nodes (destinations of a packet), cost of a route for each destination, and the most recent node (“next partner (next hop)”) with which the node is to interact for each destination are recorded in the routing table.
At a first stage for determining an optimum route in a network, each node holds only information on “which is its neighboring node” and “cost” therebetween as a routing table. Then, data on the routing tables are periodically exchanged between the nodes each other, and the routing table of each of the nodes is updated using the exchanged data. Through this operation, each node records the best “next partner” and the best “cost” for oneself with respect to any destination node in the routing table held by the node, and it is possible to determine the optimum route on the basis of the table.
In the case where a node in the network is left out, in all nodes in each of which the node is set to the “next partner”, destruction and reconstruction of the routing table is carried out. Information on the reconstructed routing table is transmitted to the neighboring nodes of the node in turn, and each node that receives the data updates the routing table of the node using the data. As a result, each existing node can find the best route for all of reachable destination nodes except for the left-out node.
In the LSA, each node broadcasts data on “which are neighboring nodes of the node” to the whole network, generates a network map by itself on the basis of data broadcasted from all other nodes, and determines the shortest route to other nodes using the generated network map by itself. Each node then generates a routing table using information on the determined shortest route with respect to all destination nodes. Each node can find the best “next partner” for the node to any destination node on the basis of the generated routing table.
Further, heretofore, there has been an audio network system in which electronic music instrument, a professional audio apparatus, a personal computer and the like are connected to a network, and audio signals for multiple channels are transferred between nodes thereof. As techniques to carry out packet communication of audio signals and the like in the audio network system, there have been known “mLAN” (for example, see the following Patent Document 1 and the like) proposed by the applicant of this application, “EtherSound” (registered trademark) disclosed in the following Patent Document 2, an asymmetric transmission network system disclosed in the following Patent Document 3, and “Cobranet” (Registered Trademark) disclosed in the following Non-Patent Document 1.    [Patent Document 1] Japanese Patent Application Laid-open Publication No. 2000-278354    [Patent Document 2] U.S. Pat. No. 7,089,333    [Patent Document 3] U.S. Pat. No. 5,764,917    [Non-patent Document 1] URL: http://www.balcom.co.jp/cobranet.htm
In a technique of a conventional dynamic routing, as described above, a routing table for determining a route in accordance with an address of a destination is required to be generated and updated in each node. Thus, a very complex process is necessary in each node, whereby it causes complexity of route control in each node. Therefore, in the conventional audio network system, dynamic routing could not have been carried out with simple control. In particular, in the audio network system, since each node is a musical apparatus such as electronic music instrument, an audio amplifier and a speaker, it is desired to carry out the dynamic routing with a simple technique without complex control.
Further, in the audio network system as described above, it is known to use a reference signal called a word clock in order to synchronize with processing timing of an audio signal (sampling clock) among a plurality of nodes. As one example, it is thought that a node (at a receiving device side) that receives a packet for transmitting an audio signal generates a word clock on the basis of timing when the packet reaches, and processes the audio signal of the received packet on the basis of the generated word clock.
Now, the larger a size of a packet for transmitting an audio signal among respective nodes is, the better transfer efficiency of the audio signal becomes. However, in the case where a word clock is generated in a node that receives the packet (at the receiving device side) on the basis of timing when the packet reaches when the packet size is set so as to be larger, there has been a defect that stability of the word clock becomes lower, and lag time between the word clock of a node that transmits the packet (at a transmitting device side) and the word clock of the receiving device side becomes larger.
Further, in the audio network system as described above, one packet including sample data containing a plurality of samples about audio signals for a plurality of channels was generated in the node that transmits the audio signal (transmitting device) every predetermined period of time, and the generated packet was transmitted to the node that receives the audio signals (receiving device). Note that a transmitting cycle of the packet in the audio network system becomes a timely larger value than a sampling period of the audio signal (i.e., a period of the transmitting cycle is longer than the sampling period). Therefore, a sample of a plurality of audio signals can be contained in a single packet.
For this reason, the larger a size of a packet for transmitting audio signals among respective nodes is, the more samples can be included in the packet every transmitting cycle of the packet. Thus, transfer efficiency of audio signals becomes good. However, there has been a problem that, in the case where a transmission error of a packet occurs, the audio signal for one packet in which the transmission error occurred is lacking, and a period to output “silent” at the receiving device side is to be generated. The larger the packet is (that is, as it is laid out with the emphasis on transfer efficiency), a silent period at a transmission error becomes longer.
Further, in the conventional audio network system, any node in the network system became a management node for managing topology (network structure) of the network system. In one example of the conventional method of detecting topology, the method includes: detecting a node connected to each port of each node in the network system; informing the management node of its detection result; making up reports (detection results) received from the respective nodes in the network system by means of the management node; and detecting topology (network structure) of the network system on the basis of the result thus made up. Namely, in the conventional method of detecting topology, there has been disadvantage that complicated processes must be carried out in which each node detects other nodes connected to ports thereof and the management node is informed of its detection result. In particular, since each node in the audio network system is musical instrument such as electronic instrument, an audio amplifier and a speaker, it is preferable that detection of topology can be carried out with a simple technique without requiring complicated control.